Want to try Xen and the Art of Debian? HowtoForge has made a perfect tutorial for you how to set up Xen on Debian.
Read all about the setup here.

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This tutorial provides step-by-step instructions on how to install Xen (version 2) on a Debian Sarge (3.1) system. It should apply to Ubuntu systems with little or no modifications.

Xen lets you create guest operating systems (*nix operating systems like Linux and FreeBSD), so called “virtual machines” or domUs, under a host operating system (dom0). Using Xen you can separate your applications into different virtual machines that are totally independent from each other (e.g. a virtual machine for a mail server, a virtual machine for a high-traffic web site, another virtual machine that serves your customers' web sites, a virtual machine for DNS, etc.), but still use the same hardware. This saves money, and what is even more important, it's more secure. If the virtual machine of your DNS server gets hacked, it has no effect on your other virtual machines. Plus, you can move virtual machines from one Xen server to the next one.